Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. Ion implantation may be performed by a beamline ion implanter or a plasma doping implanter. In a beamline ion implanter, a dopant gas may be ionized in an ion source and ions may be extracted from the ion source and accelerated to form a beam of desired energy. The ion beam may then be directed at a front surface of a wafer supported by a platen. The wafer may be a semiconductor wafer and the energetic ions in the ion beam may be embedded into the crystalline lattice of the semiconductor wafer to form a region of desired conductivity after a later anneal step. A plasma doping implanter positions a wafer supported by a platen in a plasma chamber. Plasma may be generated in the plasma chamber and ions from the plasma are accelerated towards the wafer, e.g., by biasing the wafer and/or the plasma chamber.
Most platens are electrostatic chucks that utilize electrostatic forces to clamp the wafer to a clamping surface of the platen. The clamping surface of the platen may become dirty over time due to deposits of ion beam byproducts, e.g., due to arsenic deposition in one instance when the ion beam is run into the dose Faraday cups for prolonged periods, accumulating on the clamping surface.
Deposits on the clamping surface of the platen are particularly problematic with hot ion implants or hot implants. Integrated circuit manufacturers are experimenting with hot implants for their next generation device development, e.g., fin field effect transistors (FinFETs). Hot implants typically occur at temperatures greater than 150° C. Some hot implants occur at greater than 300° C. and others occur between 300° C. and 750° C. In addition to typical deposits from ion beam byproducts, some deposits are more attracted to the hot clamping surface. In addition, thermally activated reactions can occur on the hot clamping surface of the platen to create additional deposits during hot implant processes. Trace amounts of materials such as carbon, fluorine, tungsten, and hydrogen may be present in an ion implanter. These materials can contribute to the thermally activated reactions to create additional unwanted deposits on the clamping surface of the platen. One example of this is decomposition of refractory metal from gaseous precursors. Another example is decomposition of carbon from a reaction of methane.
The accumulation of deposits on the clamping surface of the platen adversely impacts the clamping force provided by the platen. The deposits can shunt the electrostatic fields that hold the wafer. If not cleaned, the accumulation of deposits may reach an excessive level causing an inadvertent wafer drop and wafer breakage. A sputter clean process where low energy ions are directed to the clamping surface of a platen may be used to effectively clean the clamping surface. However, it is difficult to predict when a sputter clean process is necessary as the amount and location of deposit accumulation on the clamping surface depends on many variables. This may lead to excessive interruptions of ion processing for sputter cleaning. This degrades throughput performance or the number of wafers that can be processed over a given time interval and hence adversely impacts the cost of ownership. Alternatively, if not cleaned frequently enough, a risk of a wafer drop and breakage is present. In addition, it is difficult to monitor the amount of deposits in situ as the ion implanter is processing wafers.
Accordingly, there is a need in the art for monitoring the platen surface for an indication of excessive deposit accumulation.